Conveying devices and methods for controlling such conveying devices of the kind initially specified are used to convey objects, such as pallets, boxes, suitcases, postal items, and the like, in intra-logistic applications. The conveyor lines involved are composed of multiple conveyor segments arranged one after the other in series. Each conveyor segment has a conveyor drive that can be driven individually, for which purpose each conveyor segment is equipped with a control unit. In principle, each conveyor segment can have a separate control unit disposed in a housing, or the control units for several neighbouring conveyor segments may be combined in a common control module.
The control units or control modules are interconnected via bus communication. This bus communication is used to transmit signals relating to the conveying of objects on the conveyor line of conveying devices. It is possible, in particular, for signals to be transmitted that signal the presence of an object in a particular conveyor segment. Such a presence signal or busy signal can be detected by means of a sensor, for example, such as a photoelectric barrier that is connected to the control unit of the conveyor segment. Drive signals for activating a conveyor drive are also transmitted via bus communication, and other signals, for example, for merges, diverts, elevators and the like, can also be transmitted, if need be.
Equipping such conveying devices with a central controller is basically known from the prior art. In such a central control system, a centrally arranged controller, for example a PLC controller, is responsible for controlling the conveying process within the conveying device or within a section of the conveying device. For that purpose, the central control device sends commands to the individual control units via bus communication, and those commands typically include an activation signal for the conveyor drive, a deactivation signal for the conveyor drive, parameter specifications for the speed of the conveyor drive, and the like. For its part, the central controller receives signals, for example a “busy” signal, from the control units. In this kind of central control system, all the logical handling of the conveying process is performed in the central controller, and the individual control units merely serve to pass on signals and to modulate signals, where relevant.
In addition to this centralised form of control, a decentralised form of control is also known. In a decentralised control system, logical handling of the conveying process is delegated to the individual control units. To that end, the control units exchange signals among each other, for example, neighbouring control units may be informed that the next downstream conveyor segment, at least, is busy or is ready to receive goods. Depending on the kind of signal thus received about the neighbouring downstream conveyor segment being busy or having an uptake capacity, a control unit activates the conveyor drive connected to it, in order to transport the objects over the conveyor line without collisions occurring.
To prevent damage to the conveyed objects, a zero-pressure conveying mode is preferably controlled. This means that the conveyed objects never come into contact with each other on the conveyor line, thus preventing a downstream object from being damaged by the potentially strong forces that can result from the addition of conveying forces in several objects abutting each other. In a zero-pressure conveying mode, a basic distinction is made between different operating modes, namely single take-off mode, in which an object is conveyed from one conveyor segment to the next downstream conveyor segment as soon as the latter has forwarded the object lying on it, and block take-off mode, in which a conveyor segment activates its conveyor drive as soon as the conveyor drive of the neighbouring downstream conveyor segment is activated. In principle, these two operating modes can be implemented with a centralised form of control and also with a decentralised form of control. In order to control the conveyor drives in accordance with these two operating modes, or in accordance with some other type of conveying which is implemented as a mixed mode or a completely different kind of control, it is necessary for signals, such as control commands, busy signals and the like, to be communicated via bus communication.
A bus protocol is a basic requirement for bus communication. The bus protocol stipulates the form in which signals must be sent via bus communication, so that they arrive reliably at a recipient, can be interpreted by the recipient as a signal addressed to it, and so that the information contained in the signal can be read out. Central controllers are basically available in different variants that use different forms of control and which operate according to different bus protocols. Some examples of commonly used bus protocols are TCP/IP, Profibus, and CAN.
The basic choice between centralised control and decentralised control, and between the different variants of centralised and decentralised control, which entail logistics processes being shifted to a greater or lesser extent from a central controller to a decentral control unit, between different control modes in the form of single take-off, block take-off or mixtures thereof, and between the different ways of communicating with different bus protocols provide enormous variety and design freedom in the composition of intra-logistic conveying systems, which is initially advantageous, in principle. Within a defined system, in which the control hierarchy, the manner of control, and the communication protocol are all defined, the system specifications necessitated as a result allow control units to be produced that are customised for the respective system and therefore optimised with regard to production costs. Due to the very large number of control units that must be installed in larger conveyor systems, these control units account for a significant proportion of the total costs of the conveyor system and are subject as a consequence to considerable pressure to minimise costs.
One disadvantage of the enormous variety that conveyor systems fundamentally show, however, is that system components from different conveyor systems are interchangeable to only a minimal extent, if at all. It is generally impossible for the user of such conveying devices to combine components of conveyor systems made by different manufacturers, or it is necessary for components to be individually adapted. There are often incompatibilities even between components of different generations of conveyor systems made by one and the same manufacturer. The consequence is that, although customised and therefore cost-efficient, components such as the control units, conveyor drives, and any central controllers that may be required can be produced and are available for each conveyor system, this involves extensive storage facilities provided for the individual system components, to ensure that replacements for defective components, or additional components required for the expansion or restructuring of existing systems, can subsequently be supplied. Such extensive storage facilities and the concomitant need to provide production capacities for a large number of different conveyor systems, causes additional costs for the manufacturers of such conveying devices. This stands in contradiction to the actual aim of providing such conveying devices cost-efficiently.